These trannys have a lot of problems. Usually, it is some bonehead who added a different ATF fluid than the recommended ATF4+. You MUST use ATF4+ or you'll destroy the tranny. I suggest you invest in a rebuilt unit since a used unit is more likely to be on its last legs unlike other trannys. If you must go with a used unit, drain and fill the tranny with the ATF4+ fluid right away and then do another drain, ATF refill and filter replacement after a few 100 miles.

Have you replaced the converter and modulator valve. A bad converter can kill a good transmission. Also if you replaced with a newer model transmission which is electronicyou may have to also upgrade computer since newer models get shift instructions from computer.Hope this helps.Thanks